1. Field of the Invention
The present invention relates to an improved system for generating indexing signals for a beam indexing type of color cathode ray tube.
2. Description of the Prior Art
In view of the growing development of high definition television (HDTV), there is a need to develop a color CRT that can generate high resolution and high brilliance images for direct viewing and for single tube projection HDTV systems.
The current method used to generate color images in a color CRT for consumer TV employs three electron guns, a shadow mask, and a screen consisting of arrays of red, blue, and green phosphor dots arranged such that each electron gun will direct electrons to dots of a specific color. In this way, separate primary color images are generated which, when combined, produce the color picture. The obvious disadvantage of this scheme is in the use of three electron guns and in the shadow mask which typically absorbs eighty percent of the electrons from the guns, severely limiting the maximum possible brilliance of the image. Attempting to increase the brilliance of the image by raising the intensity of the electron beams results in the generation of x-rays and in localized heating of the shadow mask metal causing expansion and a loss of accurate targeting of electrons on the phosphor dots. Basically, the shadow mask type of CRT cannot be refined to meet the resolution and brilliance requirements of HDTV.
U.S. Pat. No. 3,081,414 to Goodman discloses a color cathode ray tube that eliminates the three gun/shadow mask principle and which can be used for direct viewing or in a single tube projection system. However, the Goodman system is not suitable for HDTV because the CRT's using the Goodman principle cannot be made in larger sizes, i.e. larger than an eight by eight inch screen size, and individual primary color intensities of the image cannot be adjusted by external electronic controls to produce an accurately balanced color picture or a true black and white picture.
A major problem in previous beam indexing systems, such as that disclosed in Goodman, is that the modulation of the electron beam by the video signals caused phase errors in the indexing signals and color fringing of the picture displayed on the screen, in effect reducing the resolution and brilliance of the picture displayed by the CRT. In addition, in those Goodman type systems which use ultraviolet or x-ray emissive strips on the screen or on the back of a metalized layer, the radiation pulses must be detected by an electron-multiplier or other photosensitive device capable of detecting radiation on all parts of the screen equally, without regard to the size of the screen. On larger tube sizes, this is virtually impossible to accomplish.
What is desired, therefore, is to provide a system using the Goodman principles but wherein the CRT can be made in larger sizes, the primary color intensities can be accurately adjusted and the picture displayed on the CRT has increased resolution and brilliance.